By virtue of its high reactivity, isocyanic acid represents a valuable C-1 structural unit for the synthesis of a large number of compounds.
It can be prepared according to EP-A-0 124 704 by heating molten urea in a fluidized bed to give an isocyanic acid/ammonia mixture. However, isolation of the isocyanic acid from this mixture presents difficulties because ammonium isocyanate, which very readily isomerizes to urea again, is formed when the ammonia/isocyanic acid mixture cools.
The thermal decomposition of urea with the elimination of ammonia is known from Chemical Abstracts, vol. 81 (1974) 172444m and vol. 82 (1975) 173135u, but a solid, namely cyanuric acid, is formed which has to be decomposed to isocyanic acid at 330.degree. to 600.degree. C. The decomposition proceeds only slowly and not to completion. Although EP-A-0 416 236 describes improved processes for the separation of ammonia from isocyanic acid/ammonia mixtures by the addition of tertiary amines or ethers, the object of the present invention was to find a novel process for the preparation of isocyanic acid which is easy to carry out and in which isocyanic acid is obtained in high yield and purity without a subsequent separation step.